In two-wire telephone communication systems, a pair of wires, termed tip and ring, respectively, carry a DC current which may be used for powering devices, such as telephones, coupled across the wires. Tip and ring also carry actual telephone signals, such as voice or modem signals. A voltage source at the central office of the telephone company provides a voltage across the tip and ring nodes so as to provide the aforementioned DC current. The wires connecting the telephone to the central office, of course, have a finite impedance. Accordingly, the length of wire between the central office and a particular telephone dictates the actual tip-to-ring voltage and current that the telephone receives. Thus, a telephone that is close to the central office (has a short distance of wire therebetween) normally will receive a higher voltage and therefore greater current than a telephone which is coupled to the central office through a longer wire connection.
Certain international standards, for example, international standard TBR 21 of the European Telecommunications Standard Institute (ETSI) require that the tip-to-ring current through any telecommunication device not exceed a specified current level. TBR 21, for instance, specifies a maximum tip-to-ring current of a telecommunications device of 60 milliamps. However, the standard also requires that, below 60 milliamps, the telecommunication device provides a constant impedance to the tip-to-ring line such that tip-to-ring current through the device will vary linearly with tip-to-ring voltage presented to the device.
Although many factors contribute to the establishment of such standards, one of the primary reasons for specifying a maximum tip-to-ring current is to reduce the power which the central office must generate in order to provide the DC tip-to-ring current to telecommunication devices coupled to the central office.
International standard TBR 21, as of the writing of this specification, has not been officially adopted, but is being highly recommended.
In view of international standard TBR 21 and other standards containing tip-to-ring current limit specifications, telecommunication equipment must be provided with front end DC termination circuitry for limiting tip-to-ring current through the device. Such circuitry usually takes the form of a current limit circuit that provides a constant resistance up to the specified current limit and thereafter provides a constant current at the specified current limit as the voltage continues to rise. For example, from 0 to 60 milliamps, tip-to-ring current through the device increases linearly with tip-to-ring voltage. However, when the current reaches 60 milliamps, the current will remain constant at 60 milliamps as the tip-to-ring voltage increases further.
Telecommunication devices commonly employ a current sink that may dissipate excess power when the tip-to-ring currents are high. Such current sinks may require a heat spreader or some other means of dissipating the heat buildup in the active components (commonly termed a heat sink) in order to avoid heat damage to the current sink. Heat sinks consume a significant amount of volume. Many telecommunication devices, for example, PCMCIA card modems are extremely small. Thus, the need for large heat sinks is a significant problem since it will reduce the space available for other desirable electronic componentry.
A tip-to-ring voltage of 60 volts, for example, would not be unusual in most countries. Accordingly, with a tip-to-ring voltage of 60 volts and a tip-to-ring current of 60 milliamps, up to 3.6 watts of power will be dissipated in the telecommunications device. 3.6 watts is a significant amount of power to dissipate, thus requiring a fairly large heat sink. In a compact piece of equipment in which various electronic circuits are closely packed together, the heat put out by the heat sink might adversely affect the operation of, or even destroy, neighboring electronic circuitry.
Accordingly, it is an object of the present invention to provide an improved DC termination device.
It is another object of the present invention to provide a DC termination device which can meet the performance requirements of various standards.
It is a further object of the present invention to provide an improved DC termination device for a telecommunications device which can meet the specifications of various standards and minimize power dissipation in the device.